How to sum two fields in AngularJS and show the result in an label?

The Columbus's egg is: double negation.

The initial value will be 0 (instead of null), and the result will be a sum (instead of a concatenation, because of the implicit numeric cast).

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js">
</script>

<div ng-app>  
  <input ng-model="first"  placeholder="First number"  type="text" />
  <input ng-model="second" placeholder="Second number" type="text" />
  <h1> Sum: {{first--second}}! </h1>
</div>

Have you actually created a parseFloat method in your controller? Because you can't simply use JS in Angular expressions, see Angular Expressions vs. JS Expressions.

function controller($scope)
{
    $scope.parseFloat = function(value)
    {
        return parseFloat(value);
    }
}

edit: it should also be possible to simply set a reference to the original function:

$scope.parseFloat = parseFloat;

I would also expect it to work with filters, but unfortunately it doesn't (might be a bug, or i've misunderstood how filters work):

<label>{{ (Property.Field1|number) + (Property.Field2|number) }}</label>

A workaround would be to use multiplication for casting:

<label>{{ (Property.Field1 * 1) + (Property.Field2 * 1) }}</label>

The easiest and best way to sum two numbers is using HTML5's type="number". If you do this, the inputs' values are, by default, integers.

Updated fiddle